Thermoplastic prepreg is used to make structural parts for various devices strong, rigid, and lightweight. Thermoplastic prepreg is the material resulting from impregnating fiber reinforcements with a formulated resin. These advanced composite materials offer many advantages over conventional steel and aluminum since composite parts fabricated from thermoplastic prepreg materials are generally stronger and stiffer than metals. Components fabricated from thermoplastic prepreg materials also provide greater resistance to fatigue, creep, wear and corrosion than metals.
In use, several thermoplastic prepreg plies with different fiber orientations are assembled into layers and two layers are stacked on top of each other to form a layup. The layup is then cut into thermoplastic prepreg segments. The thermoplastic prepreg segments are then assembled into a kit or wedge to form a particular structural part of the device under construction. Assembly of the kits or wedges requires stacking the thermoplastic prepreg segments in sequence and orienting the pieces according to a geometry envelope. By cutting the thermoplastic prepreg segments from layups, the structural parts of the device receive strength in more than one direction. Composite parts made from thermoplastic prepreg have very high strength in the direction of the fibers and very poor strength in other directions.
Layups are useful because they reduce the amount of time required to tailor the architecture and to catalog the thermoplastic prepreg segments of a particular device part. However, since thermoplastic prepreg material has a very low coefficient of friction, or no tackiness, the plies tend to slide, making stacking, cutting, and assembly extremely difficult. The thermoplastic prepreg plies forming a layup must be retained in alignment during and after cutting. If the thermoplastic prepreg plies are not held together during all stages of assembly, it becomes difficult and labor intensive.
After cutting thermoplastic prepreg segments from a layup, it is advantageous to retain the thermoplastic prepreg segments together in alignment during assembly of the pieces into a kit. Some thermoplastic prepreg segments are bulky and some are very small, so working with thermoplastic prepreg segments that are properly held together greatly reduces assembly time and difficulty. Each thermoplastic prepreg segment must be fastened to corresponding pieces as they are stacked and oriented as part of a kit or wedge. Several kits or wedges are typically molded together to form a composite part, such as a cylinder for a sabot.
One attempt to solve the problem of holding plies during layup assembly and cutting involved melting the thermoplastic prepreg plies together. However, conventional cutting methods have proven unsuitable for cutting such layup. If the layup is too thin, severe warping can occur during cutting.
Yet another unsatisfactory attempt to solve the problem of holding plies in alignment during layup assembly and cutting used welding the thermoplastic prepreg plies together with standard methods. Unfortunately, due to sustained high heat exposure, too many welds, and/or improper placement of welds, existing welding methods often cause thermal defects to the thermoplastic prepreg. Examples of such defects include undulation next to the weld seams, warping and shrinking, flaws and the like.
Also lacking in the prior art is an adequate method for identifying segmented pieces of the layup during assembly into kits or wedges. Nor has an adequate method been found for retaining segmented pieces in alignment for molding once they are stacked and oriented as part of a kit.